1. Field of the Invention
This invention relates to a zoom lens system, and in particular to a compact zoom lens system suitable for a lens shutter camera.
2. Description of the Prior Art
The embodiments described in Japanese Laid-Open Patent Application No. 128911/1981 (U.S. Pat. No. 4,682,860) are known as zoom lens systems proposed for such use. Each of these is comprised of a first lens unit comprising a positive lens unit and a second lens unit comprising a negative lens unit, and has been designed such that from the wide angle end to the telephoto end, each unit is moved from the image surface side toward the object side to thereby change the focal length of the entire system and keep the image surface constant. That is, they have been so constructed that a magnification change is accomplished by movement of the second negative lens unit and correction of the position of the image surface is accomplished by movement of the first positive lens unit. Generally, as a feature of such construction, the full optical length of the telephoto end which is the maximum dimension of the entire system is substantially determined by the amount of movement of the second negative lens unit.
Accordingly, for the compactness of the lens system which has been increasingly desired in recent years, it is desirable to shorten the focal length of the second negative lens unit and reduce the amount of movement of the second negative lens unit. However, taking such measures has led to the disadvantages that as the imaging performance of the lens system, the fluctuations of spherical aberration, coma and astigmatism during zooming increase and that the system difference type distortion occurring in the entire zoom range becomes great. Accordingly, it has been a difficult task to make the lens system more compact without deteriorating the imaging performance thereof. On the other hand, as new forms of the zoom lens, variable focal length lenses are proposed in Japanese Laid-Open Patent Application Nos. 10618/1984, 116710/1984, 116711/1984 and 116712/1984. However, there has been made no proposal as to a power arrangement suitable particularly for a zoom lens system for a lens shutter camera and it has been difficult to obtain any specific construction in the aforementioned propositions.
On the other hand, an optical system using a lens whose surface shape, particularly curvature, is variable is proposed in Japanese Laid-Open Patent Application No. 101618/1984 or U.S. application Ser. No. 737,342.